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Full text of "Handbook Of Chemical Engineering - I"

158
CHEMICAL ENGINEERING
reduced and the inlet losses for a given volume increase as the square of the decrease in inlet area due to this cause, while the interference with stream lines adds further to these inlet losses. The table below gives the results of tests on a multiblade type of fan (a) with a free and unobstructed inlet and (6) with a bearing and its support
LtO------70--------2.8
500      1000     1500     'SOOO    "2500, Wume .Cubic Feer per Minute.
FIG. 12.—Characteristics of a multiblade fan with single-inlet spiral casing.
loo—m~
Volume, Cubic Feer per Minute*
FIG. 13.—Characteristics of multiblade fans with double-inlet spiral canin^a.
mounted directly in the inlet.    The effect on the volumetric and mechanical eflicioneioH with a given number of revolutions per minute and static pressure is very noticeable.
Test	Revolutions per minute	Pressure, static	Inches of water		Cubic feet per minute	Horsepower		Mochanirnl effH'ic'wy, per  cent,
			Impact	Velocity		Air	Brake	
(a)	900	3.0	4.45	1.45	26 , 500	18.6	37.0	f>0. 3
(*>)	900	3.0	3.91	0.91	21,000	12.9	27.0	48. 0
Outlet or Discharge of Casing.—The area of the outlet of a spiral rasing is seldom made as small as the area between the wheel and spiral at the point of maximum area or where the air passes the cutoff in leaving the casing. A relatively large velocity pressure exists at this point and it is general practice in fans of small and medium size to make no attempt whatever to conserve the kinetic energy in this blast. In most installations this velocity pressure will be from 25 to 50 per cent of the total head developed. Fans in general have outlets varying from 25 per cent to 75 per cent larger than the area at the cutoff point. The expansion is abrupt, so that although some of the velocity pressure is transformed into static pressure, there is nevertheless considerable loss. If the fan discharges directly into the atmosphere the entire velocity pressure in t he outlet is lost. Most of this can be saved, however, if the outlet is fitted with a proper evase discharge piece (see Figs. 13 and 14). In larger fans such as are used for mine ventilation the evas6 chimney is started from the cutoff point and thereby conserves the larger portion of the velocity head at that point. If the fan is connected to a duct larger than the outlet it should have a long, tapered connection.